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Evidence that Thyroid Hormones Act in the Ventromedial Preoptic Area and the Premammillary Region of the Brain to Allow the Termination of the Breeding Season in the Ewe

Department of Physiology and Pharmacology (G.M.A., S.L.H., M.V., J.M.C., R.L.G.), West Virginia University, Morgantown, West Virginia 26506; Institute of Veterinary, Animal, and Biomedical Sciences (G.M.A.), Massey University, Palmerston North, New Zealand 5301; and Reproductive Sciences Program and Department of Physiology (H.J.B.), University of Michigan, Ann Arbor, Michigan 48109

Address all correspondence and requests for reprints to: Dr. Robert L. Goodman, Department of Physiology and Pharmacology, Health Sciences Center North, Medical Center Drive, Morgantown, West Virginia 26506-9229. E-mail: bgoodman{at}hsc.wvu.edu.

Thyroid hormones are permissive for various species to enter seasonal anestrus. In the ewe they act centrally to permit the onset of potent estradiol-negative feedback responsible for anestrus, but the specific sites of action are unknown. Therefore, we tested whether T₄ replacement via chronic microimplants in any of five brain areas could reverse the reproductive effects of thyroidectomy. Diffusion of ¹²⁵I-T₄ from the microimplant was largely (>98%) limited to a 1.2-mm radius. A marked decline in LH concentration in ovariectomized, estradiol-treated ewes was used as an index for anestrus. In experiment 1, all thyroidectomized (THX) ewes with microimplants in the medial preoptic area, A15 area, and medial basal hypothalamus failed to enter anestrus; instead, LH levels remained elevated, similar to those in untreated THX controls. In ventromedial preoptic area (vmPOA)-microimplanted ewes, only the two animals with the most caudal microimplants entered anestrus, as did thyroid-intact controls and THX ewes receiving icv or sc T₄ replacement. In experiment 2, all vmPOA-treated ewes with similar placements to those effective in experiment 1 along with all ewes microimplanted in the premammillary region entered neuroendocrine anestrus. Thus, the premammillary region and vmPOA are brain sites in which thyroid hormones act to permit the onset of seasonal anestrus.

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